Injection laser diodes are potentially the most efficient narrow band light sources available. Their small size and high brightness makes them ideally suited for applications as sources in ranging illuminators, fiber optics communications, integrated optics and diode pumping of Nd: YAG lasers, particularly for space applications.
The laser cavity in a laser diode is usually formed by two mirrors at the ends of a semiconductor crystal. Semiconductors usually have a high refractive index, particularly for light of photon energies usually involved in such devices, so that a large enough discontinuity exist between the semiconductor and the surrounding air to form mirrors without any coating. Semiconductor crystals cleave along planes of weakest binding so the mirrors are easily constructed to be perpendicular to the junction in a junction laser diode. A small increase in the refractive index exists in the junction area forming a dielectric waveguide that confines the radiation. In the other direction but transverse to the emission direction, small random variations in refractive index confine the radiation and form it into filaments. Stripe geometry laser diodes have an active region that can be limited to a single filament and, therefore, stripe laser diodes have characteristics that can be reproduced more easily.
Semiconductor laser diodes have numerous advantages. They are small. The dimensions of the active regions thereof typically are submicron to a few microns across with the lengthwise dimension usually no more than a fraction of millimeter. The mirrors are an integral part of the crystal structure and usually are formed by cleaving the crystal so they have high mechanical stability. High efficiencies are possible. Pulsed junction laser diodes have been operated at as much as 40% external quantum efficiency. They are versatile. For example, junction laser diodes conventionally are pumped with direct current and their output can be amplitude, frequency, or pulse position modulated into the GHz range. They can lase at wavelengths from 20 to 0.7 microns with the proper choice of semiconductor alloy and can operate in a single mode. They also can be operated continuously at room temperature. Continuous outputs of as much as 40 mW have been obtained at room temperature with much higher powers at lower temperatures.
Their universal application has been restricted, however, because severe problems have plagued laser diodes users. These problems relate to mounting strain thermal effects caused by the necessarily large heat dissipation per unit area of the diodes, and by the strain induced by thermal cycling. Moreover, laser diode efficiency and device lifetime is markedly decreased by small increases in junction temperature. Effective laser diode use in system applications has required the ability to operate diodes individually or in arrays under optimum and controllable conditions heretofore unavailable. This is because commercially available diodes in arrays use standard headers which neither mechanically yield nor are adaptable for combining matched parameter diodes in high output high density arrays. Lens design and power efficiency in various applications has been difficult because of the less than optimum available mounting configurations. Heretofore available methods for mounting laser diodes do not allow arrays to be built up from selected, parameter matched, quality laser diodes. Overall array efficiency is decreased by this deficiency as much as 50% in output power and with undesirable variations up to 200 A in optical band width. A short lifetime of some of the diodes can also result. Therefore, in addition to a better and more efficient mounting arrangement, there has been a need for a mounting arrangement which allows injection laser diodes to be individually mounted and tested and then configured into an array after selection and matching of the various parameters of the diodes.